A photodetector is an apparatus comprising a plurality of photodiodes arranged one-dimensionally or two-dimensionally, and an integrating circuit including an amplifier and an integral capacitance part and may further comprise signal processing circuits subsequent thereto (see for example, Japanese Patent Application Laid-open Gazzete No. H4-270960). With this photodetector, electric charges of amounts that are in accordance with the intensities of light made incident onto the respective photodiodes are output by the photodiodes, these electric charges are accumulated in the integrating capacitance elements, and voltages that are in accordance with the amounts of the accumulated electric charges are output from the integrating circuit. Then an A/D converting circuit converts the voltages outputted from the integrating circuit, into digital values. The photodetector is arranged to detect light incident on a photosensitive surface on which a plurality of photodiodes are arranged, based on the digital values outputted from the respective A/D converting circuits in accordance with amounts of electric charges generated in the respective photodiodes.